Fix the memory leak in nsd tests

The NsdManager constructor creates a new thread, which is not
intended to be closed. However, the test in NsdMangerTest and
NsdServiceTest create a new NsdManager instance, resulting in a
large number of threads running during the tests. This can lead
to an out-of-memory error for the tests. To resolve this issue,
use the common singleton thread ConnectivityThread to handle
tasks from the various instances.

Fix: 308544734
Test: atest ConnectivityCoverageTests --test-filter Nsd*
Change-Id: I1908a2bb1bd20da8e0498353bca26f7e89e16626